1. Field of the Invention
This invention relates to instruments for the non-invasive quantitative measurement of constituents in a product, such as protein in wheat or blood glucose levels in an individual's blood. Specifically, this invention relates to LED/IRED near-infrared instruments having improved measurement sensitivity.
2. DESCRIPTION OF THE BACKGROUND ART
Since the early 1970's, near-infrared quantitative measuring instruments have become widely used in agricultural applications. Such instruments have proven to provide highly accurate quantitative and qualitative measurements of chemical constituents for a wide range of products.
An example of these near-infrared quantitative measuring instruments having agricultural application is the TREBOR-90XL grain analyzer. The TREBOR-90XL is a near-infrared diffuse transmittance instrument which provides highly accurate quantitative measurements, such as moisture content in grain.
Near-infrared quantitative analysis instruments have also been used to obtain information concerning the chemical composition of blood. This information has been used to assess the health characteristics of both people and animals. For example, analysis of the glucose content of blood provides an indication of the current status of metabolism. Blood analysis, by the detection of above or below normal levels of various substances, also provides a direct indication of the presence of certain types of diseases and dysfunctions.
A current type of blood glucose analytical instrumentation has been developed which non-invasively measures blood glucose levels in individual users. A near-infrared quantitative analysis instrument of this type is described in U.S. Pat. No. 5,077,476 (Rosenthal). The non-invasive blood glucose measurement instrument analyzes near-infrared energy following interactance with venous or arterial blood, or transmission through a blood-containing body part. The instrument measures a change in light absorption that occurs, in part, due to the glucose content of the blood stream.
Non-invasive measurement instruments of this type have broad applications for the diabetic community. For example, people with diabetes have wide changes in their blood glucose content during the day which often require multiple measurements per day for good disease control. The ability to make these near-infrared blood glucose level measurements non-invasively means that more measurements will likely be made per day than would be made using the more painful blood drawing approach.
The near-infrared quantitative measurement instruments discussed above both require the capability to take measurements over a broad range of light levels. For example, measurements are taken of highly transparent materials, i.e. where the Log 1/T value is near zero, and measurements are taken of nearly opaque materials, i.e. those having a Log 1/T values approaching 7.0.
The commercial near-infrared analysis instruments have essentially used two different techniques to perform such low light measurements over a broad optical range. A first technique involves having the detector signal linearly amplified and then converted to a logarithmic function by means of a logarithmic amplifier. The output of the logarithmic amplifier is then converted to a digital signal via an analog-to-digital ("A/D") converter for entry into the microprocessor built into the instrument. An alternative approach involves omitting the expensive logarithmic amplifier, which normally has undesirable nonlinearity characteristics, and using a higher resolution A/D converter, i.e. an A/D converter having a significant number of additional "bits" of resolution, in order to provide the requisite data resolution for the microprocessor.
For example, a 12 bit A/D is used if the signal is first modified by a logarithmic amplifier. If there is no logarithmic amplifier, typically a 16 bit A/D converter is used. However, instruments utilizing the logarithmic amplifiers or high resolution A/D converters increase the instrument's overall cost. Specifically, logarithmic amplifiers, such as Analog Devices, Inc. model 755N, cost in excess of $80. Likewise, the difference in cost between a high speed 12 bit and a 16 bit A/D converter can be in approximately the same price class.
Thus, there is a great need for a near-infrared measurement instrument which provides the resolution necessary for precision measurement using near-infrared techniques but without the expensive electronic circuitry currently necessary to achieve high resolution measurements.
There is also a need, as the calibration of near-IR instruments becomes customized for an individual subject, to ensure that the instrument is being used to analyze only the subject for whom (which) it has been calibrated.